If an animal has a backbone it also has a brain. Dogs, lizards, frogs, fish: even birds have brains. But none of these creatures demonstrate the same capacity for learning, language, emotion and abstract thought that distinguishes the human species. What is it about the human brain that gives us the edge? Neuroscientists (scientists who study the brain and nervous system) and philosophers have learned plenty about the functioning of the brain. But they admit there are aspects of brainpower that remain among humanity's most enduring mysteries. An introduction to the brain
The basic facts about the brain are well known. Weighing in at around 1.3 kilograms, it is one of the largest organs in the human body. It is nothing remarkable to look at – a wrinkled object about the size of a number 13 chicken – but it consists of a complex and apparently hopelessly tangled mass of nerve cells, or neurons. It sits inside the skull immersed in a fluid that cushions it from sudden impacts to the head. Neurons are the basic unit that makes up the brain and nervous system. They are specialised cells that act like telegraph wires carrying messages in the form of electrochemical impulses throughout the body. These impulses travel very quickly, although not as quickly as an electric current would travel: it takes about one hundredth of a second for a pain in your little toe to register in your brain. This is quite remarkable, given that the impulse travels a complex path through many neurons and across the gaps (synapses) between neurons to reach its destination (Box 1: The human nervous system). Brain functions
The brain performs a number of functions, many of which are related to the physical needs and actions of the body. For these functions, the brain can be thought of as the command centre of the human nervous system, much like the headquarters of a military unit. It receives information from its vast network of neurons throughout the body. Based on this information, it makes decisions and issues commands that stimulate muscles and give the body movement. Other brain functions are more like those of a university than a military headquarters. These functions give us the ability to read, write, talk and think about issues more broad than where the next meal is coming from. Structure of the brain – an overview
The brain is shaped like two fists standing side by side on a single wrist. The 'wrist' is the brainstem, connecting the brain to the spinal column, and the 'fists' constitute the left and right hemispheres of the largest part of the brain, the cerebrum. At the back of the brain, below the cerebrum, is the cerebellum: its main function is to synchronise the muscles of the body.
The cerebral cortex: Control centre
The cerebrum has an outer layer of grey matter arranged in folds. This layer, the cerebral cortex, is just a few millimetres thick but because of its numerous folds constitutes 40 per cent of the entire brain mass. Different areas of the cerebral cortex play specific roles in human thought and activity. For example: the frontal lobes control behaviour, intellect and emotion; the speech area controls talking; specific sections of the motor area control voluntary muscles in different parts of the body, and so on. In general, the right side of the brain controls movement in the left side of the body and the left side controls the right. However, there is some specialisation. For example, language is more a function of the left hemisphere and recognition of shapes is more a function of the right (Box 2: Functions of the left and right sides of the brain).
Humans have large brains When body weights are taken into account, the brain is much larger in mammals than in other vertebrates and reaches its greatest size in monkeys, apes and humans. The unusual size of the cerebral cortex in the human brain may partly explain its unique abilities. If the cerebral cortex of a frog is damaged or destroyed, there is no obvious change in the behaviour of the animal. A rat without a cerebral cortex can still move about. Human beings, though, are totally paralysed and unable to see, although internal functioning continues. More to learn
It is not so long ago that the only way scientists could study the human brain was to dissect it after a person died. Now scientists and clinicians have access to several imaging techniques that open a window on the living, conscious brain. These techniques are powerful tools for research into normal brain function and for locating tumours or blocked blood vessels in the brain (Box 3: Brain imaging). Australian research Australian neuroscience boasts a Nobel laureate – the late Sir John Eccles. He was awarded the prize in 1963 for research that explained how impulses were transmitted between neurons. Eccles was also the first to record electrical signals from the interior of neurons within the central nervous system. Australian researchers have continued to be leaders in the field of neuroscience, particularly in the area of neurotransmitters, the chemical messengers that convey impulses between neurons. Their studies have led to advances in the treatment of neurological diseases and may lead to the development of drugs that help improve memory (Box 4: Neurotransmitters and drugs). Boxes 2. Functions of the left and right sides of the brain 4. Neurotransmitters and drugs
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Posted May 1998.
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This topic is sponsored by the Howard Florey Institute of Experimental Physiology and Medicine.
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